1. Field of the invention
This invention relates to a stage device for positioning a workpiece to be machined, for example, and to an exposure apparatus equipped with this stage device and used in transferring a mask pattern onto a substrate in a lithographic process for manufacturing semiconductor devices, liquid crystal display devices, thin film magnetic heads, and so forth. The present invention is particularly suitable for use in exposure apparatus having various mechanisms such as a mechanism for measuring imaging characteristics.
2. Description of the Related Art
High exposure precision is required of a batch exposure type (stepper type) or scanning exposure type (step-and-scan type) of exposure apparatus used in the manufacture of semiconductor devices and the like. Accordingly, in the conventional exposure apparatus, a movable mirror would be fixed to the side surface of a reticle stage for carrying and positioning the reticle serving as a mask, or to that of a two-dimensionally moving wafer stage for carrying the wafer serving as a substrate, and a measurement beam would be directed at this movable mirror from an interferometer such as a laser interferometer, the result of which was that the amount of movement of the stage was continuously measured at all times, and the positioning of the stage could be carried out at high precision on the basis of the measured values. Usually, with such a stage device, interferometers on three axes have been used to measure displacement in three degrees of freedom, consisting of the two-dimensional movement component and the rotation component of the movable stage.
With such a conventional stage device, however, measurement beams from the respective interferometers had to be constantly directed at the movable mirror in all regions of the maximum movement range (movable range) of the movable stage, and because of this, the movable mirror had to be made larger than the movable range so that it would continue to reflect the measurement beams from the interferometers even if the movable stage moved.
Consequently, a large movable mirror was needed if the movable range of the movable stage was to be expanded, and this inevitably led to the overall size of the stage being larger, which was a problem in that the stage was heavier and therefore more difficult to move quickly. Also, machining a large movable mirror to the required degree of flatness entails tremendous technical difficulty, and fixing the movable mirror to the side of the movable stage without causing the mirror to bend also presents daunting technical difficulty. Unfortunately, a decrease in the flatness of the movable mirror translates directly into a decrease in positioning precision of the stage by interferometer, so ultimately there is no choice but to limit the movable range of the movable stage.
A stage device intended to solve this problem is disclosed in Japanese Patent Application Laid-Open No. 7-253304, for example. With this disclosed stage device, the number of interferometers used is greater (such as four axes) than the number of degrees of freedom in the displacement of the movable stage (such as three degrees of freedom), which allows the freedom of movement of a given stage to be measured by the remaining interferometers even if the measurement beam from one interferometer should be outside the measurement range of the movable mirror. Once the movable mirror moves back into the measurement range of this one interferometer that misses the movable mirror, the measurement values from the remaining interferometers are set as the initial value for this one interferometer, which allows the amount of movement of the movable stage to be measured by this one interferometer, and makes the movable mirror smaller than the movable range of the movable stage.
Also, with these conventional exposure apparatus, the exposure always has to be performed with the proper amount of exposure light and with the imaging characteristics maintained in a favorable state, so a measurement apparatus for measuring the state of exposure light irradiation and the like or for measuring the imaging characteristics, such as the projection magnification, is provided to the reticle stage on which the reticle is positioned or to the wafer stage on which the wafer is positioned. Examples of a measurement apparatus provided to a wafer stage include an irradiation quantity monitor for measuring the incident energy of exposure light incident to the projection optical system, and a spatial image detection system for measuring the position, contrast, etc., of a projected image. An example of a measurement apparatus provided to a reticle stage is a reference plate on which is formed an index mark used for the imaging characteristic measurement of the projection optical system.
With the above-mentioned conventional exposure apparatus, a measurement apparatus provided to the reticle stage or the wafer stage was used to achieve the proper amount of exposure and to maintain higher imaging characteristics. Nevertheless, today's exposure apparatus also need to increase the throughput (productivity) of the exposure process in the manufacture of semiconductor devices and the like. Ways of increasing throughput include increasing the exposure energy per unit of time, and raising the drive speed of the stage so as to shorten the stepping time with a batch exposure type or shorten the stepping time and the scanning exposure time with a step-and-scan type.
To thus increase the drive speed of the stage, a drive motor with a larger output is used if the size of the stage system is not to be changed, and conversely, to increase the drive speed with a drive motor of the same output as in the past, the stage system must be made more compact and lightweight. In the former case, however, the use of a drive motor with a larger output increases the amount of heat generated from the drive motor. This increase in the amount of heat causes subtle thermal deformation in the stage system, which could conceivably preclude obtaining the high positional precision required of the exposure apparatus. In view of this, the latter case, namely, making the stage system as compact and lightweight as possible, is the preferable way to increase drive speed.
Particularly with a step-and-scan type of exposure apparatus, a higher drive speed shortens the exposure scanning time and greatly improves the throughput, and another advantage is that synchronization precision between the reticle and wafer is enhanced by making the stage system more compact, as are imaging performance and alignment precision. It is difficult, however, to make the stage more compact when various measurement apparatus are provided to the reticle stage or wafer stage as in the past.
Furthermore, when a measurement apparatus for measuring the state of exposure light, imaging characteristics, or the like is provided to the reticle stage or wafer stage, a heat source such as an amp is usually attached to this measurement apparatus, and the temperature of this measurement apparatus is gradually raised by irradiation with the exposure light during measurement. As a result, there is the danger of subtle thermal deformation in the reticle stage or wafer stage and a deterioration in positioning precision, alignment precision, and so on. At present, the deterioration in positioning precision and the like caused by the elevated temperature of a measurement apparatus is slight, but as the circuit patterns of semiconductor devices and so forth become even finer in the future, the need to minimize the effect of high measurement apparatus temperatures is expected to grow.
In regard to this, the length of the movable mirror can be reduced compared to the movable range of the movable stage by using the stage device disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 7-253304, but this contributes little to making the movable stage itself more compact. Therefore, another way is needed to improve throughput in the exposure step and reduce the effect of the irradiation heat of the exposure light.
In addition to improving throughput, there is also a need to improve resolution, depth of focus (DOF), line width control precision, and so forth with an exposure apparatus, and particularly a projection exposure apparatus. When the exposure wavelength is λ and the number of apertures in the projection optical system is N.A., then the resolution R is proportional to λ/N.A., and the depth of focus DOF is proportional to λ/(N.A.)2. Accordingly, merely reducing the exposure wavelength λ and increasing the number of apertures N.A. in order to increase the resolution R (make the value of R smaller) results in a depth of focus DOF that is too small.
This necessitates the formation on the wafer of a pattern that combines a periodic pattern, such as a line and space (L/S) pattern, with an independent pattern, such as a contact hole (CH) pattern, in order to manufacture a device. As to periodic patterns, for instance, a technique for increasing resolution by narrowing the depth of focus by the so-called modified illumination method has recently been developed, as disclosed in Japanese Patent Application Laid-Open No. 4-225514. The phase shift reticle method has also been developed. Similarly, regarding independent patterns, a technique has been developed for substantially increasing depth of focus and the like by a method such as controlling the coherence factor of the illuminating light.
In light of these technological trends, double exposure is being given a second look as a method for increasing resolution substantially without making the depth of focus too deep. Specifically, when the double exposure method is applied, the reticle pattern used for a certain layer is split into a plurality of reticle patterns according to the type in question, and each pattern is exposed under its own optimal illumination and exposure conditions, which results in an overall broad depth of focus and a high resolution. Recently, this double exposure method has been applied to a projection exposure apparatus in which a KrF excimer laser or an ArF excimer laser is used as the exposure light, and there have been attempts at exposing a pattern for a device including an L/S pattern with a line width down to just 0.1 μm.
However, when this double exposure method is applied to a projection exposure apparatus having a single wafer stage, the alignment, exposure, and other such steps have to be repeatedly carried out in series, which is a drawback in that throughput suffers markedly. In view of this, there has been proposed a projection exposure apparatus with which alignment and exposure can be carried out in parallel. However, when a plurality of wafer stages are thus provided, if an attempt is made to measure the positions of the movable stages of the wafer stages with a single interferometer, then the measurement beam of the corresponding interferometer will be interrupted when the movable stages move a considerable distance, so when the various movable stages are positioned at the exposure position alternately, for example, it is difficult to position the movable stages quickly and with good reproducibility.
In light of the above, it is a first object of the present invention to provide a stage device having a plurality of functions, wherein the moving components are made smaller while still allowing these plurality of functions to be executed, so that these moving components can be moved quickly and their positions can be measured at high precision and with good reproducibility.
It is a second object of the present invention to provide a stage device with which the moving components can be quickly moved to their respective target positions with good reproducibility when a plurality of moving components are provided so that double exposure or another such method can be performed.
It is a third object of the present invention to provide an exposure apparatus equipped with such a stage device, with which the moving components for positioning the reticles or wafers can be made smaller while preserving the function of measuring the characteristics in the transfer of the reticle patterns, the imaging characteristics of the projection optical system, or the like.
It is a fourth object of the present invention to provide an exposure apparatus equipped with such a stage device, with which double exposure or another such method can be performed at a high throughput.
It is a fifth object of the present invention to provide a positioning method with which positioning can be carried out quickly using a stage device such as this.
It is a sixth object of the present invention to provide a stage device with which position measurement can be performed at high precision by interferometer method over a range wider than the movable mirror (or equivalent mirror surface), and as a result, the moving components can be made more compact, as well as an exposure apparatus equipped with this stage device, and a positioning method that makes use of this stage device.
It is a further object of the present invention to provide a method for manufacturing a device using the above-mentioned exposure apparatus.